Futuristic wastewater treatment plant battling antibiotic resistance.

Colistin Resistance: Are Our Wastewater Treatment Plants Failing Us?

Nico Varela in Health & Wellness October 2026 • 4 min read.

"New research highlights the increasing presence of antibiotic-resistant genes in wastewater, raising concerns about the effectiveness of current treatment methods and the potential spread of resistance."

In an era where antibiotic resistance is rapidly becoming a global crisis, colistin, once considered a last-resort antibiotic, is facing increasing challenges. The rise of multidrug-resistant bacteria has led to a greater reliance on colistin, but its effectiveness is now threatened by the emergence of colistin-resistant genes, particularly the plasmid-mediated mcr-1 gene.

The mcr-1 gene, which confers resistance to colistin, was initially identified in China and has since been detected worldwide. Its presence in various sources, including animals and humans, raises concerns about its potential spread and impact on public health. Wastewater treatment plants (WWTPs), which receive sewage containing antibiotic residues and resistant bacteria, could act as "hot spots" for the acquisition and dissemination of antibiotic resistance genes.

Recent research has focused on detecting and quantifying the mcr-1 gene in wastewater to assess the effectiveness of current treatment processes and understand the extent of its prevalence. By examining wastewater samples collected over different time periods, scientists aim to gain insights into the dynamics of colistin resistance and identify potential strategies to mitigate its spread.

Tracking Colistin Resistance: What the Wastewater Tells Us

Futuristic wastewater treatment plant battling antibiotic resistance.

A recent study investigated the presence and abundance of the mcr-1 gene in raw and treated wastewater samples collected from a wastewater treatment plant in Girona, Spain. Samples were analyzed from two winter seasons (2011 and 2016) to determine if there were any significant trends in the level of colistin resistance over time.

The study utilized a real-time PCR assay to accurately detect and quantify the mcr-1 gene in the wastewater samples. This method allowed researchers to determine the absolute abundance of the gene and assess the effectiveness of the wastewater treatment plant in removing colistin-resistant bacteria.

The results revealed that the mcr-1 gene was significantly more abundant in raw sewage samples compared to treated wastewater samples.
This finding suggests that conventional wastewater treatment processes reduce the number of colistin-resistant bacteria but do not eliminate them entirely.
Furthermore, the study found a significant increase in the abundance of the mcr-1 gene between the 2011 and 2016 winter seasons.
This indicates that colistin resistance is becoming more prevalent over time, highlighting the need for improved monitoring and mitigation strategies.

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Latest Research

Wastewater Can Retain Colistin-Resistant Bacteria

Lekunberri et al. (2017) examined raw and treated wastewater samples and found that treatment reduced the number of colistin-resistant strains but did not eliminate them. A review also notes that treated wastewater may still carry mcr-1 into receiving water bodies. Separately, a 2024 study reported that multidrug-resistant K. pneumoniae strains, including colistin-resistant strains, were discharged from wastewater treatment plants into aquatic ecosystems; the authors called for monitoring to genetically characterize these pathogens.

Counterpoints

Evidence and Knowledge Gaps

The environmental evidence remains fragmented, even as aquatic environments are described as possible reservoirs and dissemination pathways for colistin resistance. The concern matters because colistin is a last-line treatment for severe Gram-negative infections, including those caused by Pseudomonas aeruginosa and Acinetobacter baumannii. Resistance can compromise treatment efficacy, with a review linking it to higher treatment-failure rates, longer hospitalizations, and increased mortality. A 2024 thesis also reports a scarcity of research estimating the current load and impact of residual colistin and carbapenems in wastewater.

Comparison

Different Research Questions, Shared Treatment Challenge

The supplied sources address different comparisons and do not provide a direct head-to-head assessment of wastewater treatment performance. A 2017 study screened seven wastewater treatment plants with different population equivalents and catchment areas for the mcr-1 colistin-resistance gene. By contrast, a 2026 clinical analysis set out to compare colistin monotherapy, colistin-based combinations, and alternative antibiotic strategies for multidrug-resistant Gram-negative infections. A 2023 study focused on susceptibility patterns in colistin-resistant Enterobacterales isolates to inform alternative treatment options for critically ill patients.

The increasing prevalence of colistin resistance genes in wastewater raises concerns about the potential for these genes to spread into the environment and ultimately impact human health. Wastewater effluents can contaminate surface waters used for recreational purposes or as sources of drinking water, potentially exposing individuals to antibiotic-resistant bacteria.

The Future of Wastewater Treatment: Combating Antibiotic Resistance

The findings of this study emphasize the need for a comprehensive approach to combat antibiotic resistance, including strategies to improve wastewater treatment processes. Traditional WWTPs may not be sufficient in removing antibiotic-resistant bacteria and genes, necessitating the development of advanced treatment technologies.

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Future Outlook

Improving Wastewater Surveillance

The cited article describes a new study raising concern that current wastewater treatment plants may be insufficient at removing antibiotic-resistant bacteria. It says this could contribute to the spread of resistance genes and pose a public-health risk. The snippet does not identify the study’s methods or provide quantitative results, so it supports a call to examine treatment performance and the potential for gene spread rather than a quantified estimate of risk.

Real-World Impact

Wastewater Pathways and Seasonal Variation

A 2026 study reports that its wastewater treatment process was completely ineffective at eliminating mcr-5 genes. It also found significant seasonal differences in the prevalence of mcr-1, mcr-4, and mcr-5, with warmer and colder seasons differing in environmental samples. More broadly, municipal sewage systems and wastewater treatment plants are described as both reservoirs and pathways for the dissemination of antibiotic resistance.

Several advanced treatment methods, such as membrane filtration, advanced oxidation processes, and UV disinfection, have shown promise in removing antibiotic-resistant bacteria and genes from wastewater. However, the cost-effectiveness and feasibility of implementing these technologies on a large scale need to be further evaluated.

In addition to improving wastewater treatment, it is crucial to promote responsible antibiotic use in both human and animal medicine. Reducing the overall consumption of antibiotics can help to slow down the development and spread of antibiotic resistance. By implementing these strategies, we can protect public health and ensure the continued effectiveness of antibiotics for future generations.

About this Article -

Written with AI assistance from published research, and reviewed by the Mystum team. See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1016/j.ijantimicag.2017.08.018, Alternate LINK

Title: Detection And Quantification Of The Plasmid-Mediated Mcr-1 Gene Conferring Colistin Resistance In Wastewater

Subject: Pharmacology (medical)

Journal: International Journal of Antimicrobial Agents

Publisher: Elsevier BV

Authors: Itziar Lekunberri, José Luis Balcázar, Carles M. Borrego

Published: 2017-12-01

Everything You Need To Know

What is colistin, and why is its resistance a concern?

Colistin is an antibiotic of last resort, used to treat infections caused by multidrug-resistant bacteria. Its significance lies in its critical role when other antibiotics fail. The emergence of resistance genes, like the plasmid-mediated mcr-1 gene, threatens its effectiveness and, by extension, the ability to treat severe infections. The implications are that infections previously treatable with colistin may become untreatable, leading to increased morbidity and mortality.

What is the mcr-1 gene, and why is it significant?

The mcr-1 gene is a specific gene that confers resistance to the antibiotic colistin. It was first identified in China but has since been found globally in various sources, including animals, humans, and, as this research demonstrates, wastewater. Its importance stems from its ability to spread resistance, making bacteria impervious to colistin. The implications are serious, as the spread of mcr-1 reduces treatment options for infections, potentially leading to treatment failures and increased public health risks.

What role do wastewater treatment plants (WWTPs) play in the spread of antibiotic resistance?

Wastewater treatment plants (WWTPs) are facilities designed to remove contaminants from sewage. In the context provided, WWTPs are significant because they receive sewage containing antibiotic residues and resistant bacteria, potentially acting as hotspots for antibiotic resistance gene dissemination. This is because standard WWTP processes may not fully eliminate resistant bacteria, allowing the mcr-1 gene to persist and potentially spread into the environment via treated effluent. The implications are that WWTPs can inadvertently contribute to the spread of antibiotic resistance, thereby posing risks to public health and the environment.

How is a real-time PCR assay used in this research, and why is it important?

A real-time PCR assay is a laboratory technique used to detect and quantify specific DNA sequences, such as the mcr-1 gene, in a sample. In the context of the research, it's important because it allows researchers to accurately measure the abundance of the mcr-1 gene in wastewater samples. This helps assess the effectiveness of wastewater treatment processes in removing colistin-resistant bacteria and to track changes in resistance levels over time. The implications include the ability to monitor trends in antibiotic resistance and to evaluate the effectiveness of different treatment strategies.

What do the study's findings suggest about colistin resistance and wastewater treatment?

The study's findings emphasize that the mcr-1 gene was more abundant in raw sewage than treated wastewater, indicating that standard wastewater treatment reduces, but does not completely eliminate, colistin-resistant bacteria. The increase in mcr-1 gene abundance between 2011 and 2016 highlights that colistin resistance is becoming more prevalent. The implications are that traditional WWTPs are not enough, there is a need for advanced treatment technologies and better monitoring is crucial to combat the spread of antibiotic resistance, and to protect public health and the environment.